Gravimetric processes and manual counting processes were known in the past for the detection of yarn defects caused by impurities. Gravimetric processes are based on the collection and weighing of eliminated trash particles during the spinning process, e.g. on a rotor spinning machine. This does however not indicate how much trash still remains in the finished yarn nor the type of impurities remaining.
Also known are manual counting processes to ascertain how many trash particles are in the yarn. For this purpose the spun yarn is knit on a circular knitting machine and a certain surface is cut out of the knit material. A grid is placed on the cut-out surface and the number of trash particles in the individual areas is counted. The disadvantage of this system is the great amount of time necessary to prepare the knit material and then to count the particles. It is also a disadvantage that the success of the process depends very much on the accuracy of person doing the counting. Comparisons have shown that examinations of the same knit material by different persons show very different counting results.